1 . Field of the Invention
The invention relates to electric drive technology. It is based on a method of operating a drive system. Furthermore, the invention relates to a device for carrying out the method.
The fields of application of the invention are, for example, machine tools and rotary printing machines. A preferred application is rotary printing machines for printing newspapers, having a large number of individually driven printing cylinders and having flexible production capabilities.
2. Discussion of Background
A method and a device of the generic type is known from the text of the paper presented at the "Ifra" seminar, May 21 and 22, 1996, by Juha Kankainen, Honeywell Oy, Varkaus, Finland. Presented there is a shaftless drive system in which a plurality of drive groups are provided, each drive group comprising a drive control means and at least one drive. For its part, the drive comprises a drive controller and at least one motor. The drive controllers are connected to one another via a drive bus (in the document mentioned, designated "vertical SERCOS ring"). The drive control means (designated "process station") are connected via a dedicated drive data network in the form of a ring. The drive control means are connected to higher-order control units. According to the SERCOS standard, the drives are synchronized via a local synchronization clock.
A further concept for a shaftless drive system in the form of a rotary printing machine is disclosed by German Laid-Open Specification DE 42 14 394 A1. The rotary printing machine disclosed in this document constitutes a drive system which comprises at least two drive groups in the form of individually driven printing point groups. The drive groups have a drive control means and at least one drive, which is composed of a motor and a drive controller. The drive groups receive their position reference (master shaft) directly from the folding apparatus. The drive controllers of the drive groups are likewise connected via a drive bus. The drive control means are connected to one another via a data bus and to an operating and data processing unit. The predefinition of desired values and the administration of the printing point groups is carried out via this data bus.
The drive controllers of such a drive system enable torque control, speed control (rotational speed control) or position control (angular position control) of the driven shaft. In the case of high requirements for angular synchronism, such as there are, for example, in drive systems in machine tools and in printing machines, position controllers (angular position controllers) are preferably used.
The digital drive controllers are preferably equipped with fast digital signal processors. Such fast digital drive controllers can execute one control cycle, in the case of position control, in a very short computing time, preferably in 250 .mu.s or in a shorter cycle time.
In such drive systems, three-phase motors are preferably used. The electric drive power is supplied to the individual motor via a power electronic circuit, preferably having a frequency converter function. The power electronic circuit is driven by the digital drive controller.
The individual drives are equipped with highly accurate actual value transmitters, preferably optoelectronic position transmitters. The signal resolutions of such known, highly accurate actual value transmitters lie in the range of over 1,000,000 points per revolution (360.degree.). The practically useful measurement accuracies of the known actual value transmitters lie in the range of more than 100,000 points per revolution (360.degree.).
The actual value transmitter for the individual drive is often fitted to the motor shaft. However, arrangements are also known in which an actual value transmitter is fitted to the load driven by the motor. For example, in the case of printing machines it is advantageous to fit a high-resolution position transmitter at the torque-free end of the driven printing cylinder.
The decisive factor for high-precision synchronism of a plurality of position-controlled individual drives is the accurate synchronization of the drives via a common clock, and the cyclic supply thereto of position desired values in the predefined clock frame.
The common clock ensures that the individual drive controllers execute their position control functions exactly synchronously in time (at the same time) and in so doing evaluate the predefined position desired values in a time-consistent manner (at the same time).
Drive systems are known in which a number of individual drives are supplied with a common synchronization clock and with desired value data from a central drive control means via a fast drive bus.
The data transfer is preferably carried out according to the stipulations of the SERCOS standard. The SERCOS standard is a data interface agreed by several drive manufacturers, which supports the synchronization and the desired value transmission for the drives of a drive group.
With reference to the SERCOS standard, see: "Kurzubersicht der Produkte mit SERCOS-Interface" [Brief overview of products with a SERCOS interface], 2nd edition, October 1995, Fordergemeinschaft SERCOS interface e.V., Im Muhlefeld 28, D-53123 Bonn; or "SERCOS interface, Digitale Schnittstelle zur Kommunikation zwischen Steuerungen und Antrieben in numerisch gesteuerten Maschinen" [SERCOS interface, digital interface for communication between control means and drives in numerically controlled machines], Update 9/91, Fordergemeinschaft SERCOS interface e.V., Pelzstrasse 5, D-5305 Alfter/Bonn.
In this case, the drive bus is preferably implemented as a ring-like glass fiber connection. The data transmission is in this case controlled and coordinated by a central main station (bus master). The individual drives, connected to the ring-like data line, are substations, that is to say slaves, in the data transmission. The individual drives receive a common synchronization clock and their desired value data from the central drive control means via the drive bus. The central drive control means generates the common synchronization clock and calculates the desired values for the individual drives of the drive group. The drive control means in this case supplies, in short cycle times, in each case new desired values for the individual drive controllers. Preferred cycle times for the transmission of the common synchronization clock and for the calculation and the transmission of the desired values of the individual drives of a drive group are 62 .mu.s, 125 .mu.s, 250 .mu.s, 500 .mu.s, 1 ms, 2 ms, 3 ms, . . . 63 ms, 64 ms or 65 ms in the SERCOS standard.
Using drive systems of this type, quite high synchronization accuracies can be implemented between the drives of a drive group. Mechanical synchronization shafts and mechanical gear transmissions can be replaced by electronically synchronized groups of individual drives. Drive systems of this type, with electronic synchronization of the individual drives, thus enable electronic synchronization shafts and electronic gear transmission functions.
Using drive systems of this type; it is possible, for example, for rotary printing machines with individually driven printing cylinders to be implemented--without mechanical synchronization shafts (see, for example, the Laid-Open Specification mentioned at the beginning and the text of the paper).
Rotary printing machines for multicolor printing, having individually driven printing cylinders, place particularly high requirements on the angular synchronism of the individual drives. In the case of four-color printing, synchronization accuracies of the individual printing cylinders of the order of magnitude of 10 .mu.m are often required. In the case of a printing cylinder periphery of, for example, 1 m this means that a position measurement and position control with an accuracy of better than 100,000 points per cylinder revolution (360.degree.) must be carried out. In the case of printing speeds (paper web speeds) of more than 10 m/s, this moreover means that the time synchronization error between the individual drives of the cylinders printing on one paper web (according to the formula time=distance/speed=10 .mu.m/10 m/s=1 .mu.s) must be smaller than 1 .mu.s.
This means that the individual drive controllers, during their position control, have to be synchronized via the drive bus with a time accuracy of better than 1 .mu.s.
Using the drive systems cited and the synchronization and the supply of desired values to the individual drives via a ring-like glass fiber connection, according to the SERCOS interface agreements, these requirements can be achieved only for drive groups having a limited number of individual drives.
As a result of the central common devices, drive control means and drive bus, there are specific bottlenecks and disadvantages, however, which have an increasingly negative effect with an increasing number of drives in the drive group. The most important limitations and disadvantages are the following:
With an increasing number of drives, as a rule the synchronization error of the data line increases. In the case of a ring-like glass fiber connection with data transmission functions, for example in accordance with the SERCOS interface agreements, the following is true for the growth of the synchronization error: In the case of each drive controller which is connected to the glass fiber ring, a time-discrete signal sampling is carried out at a specific sampling period, for example 30 ns. The binary signal reproduced in the receiver as a result of time-discrete sampling can thus jitter at most by the sampling period, for example 30 ns (on the time axis, with respect to the original signal in the transmitter). Thus, in each participant, that is to say drive, there is a time-sampling error which makes itself noticeable as jitter (time jitter) This sampling error (jitter) also refers to the common synchronization clock. The sampling error therefore manifests itself as a synchronization error. The sampled signal is used in the individual drive and--following appropriate regeneration of the signal--is also forwarded to the respective next drive controller in the glass fiber ring. Depending on the number of drives in the glass fiber ring, the synchronization errors (jitter) of the individual participants add up to an overall error. As an example, in the case of 33 drives in the glass fiber ring, each having a 30 ns sampling error, this results in an overall synchronization error of about 1 .mu.s.
With an increasing number of drives in the ring, the cycle time needed for the data transmission also increases. If, for example, a data transmission time of 250 .mu.s is needed per drive, then in the case of the connection of 32 drives to a ring, this means that the cycle time for the data transmission must be at least 8 ms. A rise in the cycle time for the transmission cycles also means longer time intervals between the individual synchronization clocks--in the example cited this is 8 ms. Between successive synchronization clocks of the ring, the local clock generators of the individual drives run freely--and wander (drift) more or less from one another, depending on the inaccuracy of the crystals used.
If the local clock generator of a drive is, for example, equipped with a crystal of a quality of 100 ppm (parts per million), then this clock generator may have a time deviation of (plus or minus) 0.8 .mu.s after 8 ms, because of its inaccuracy. The time deviation between two arbitrary drives, which is caused by the inaccuracy of the two local clock generators, is the sum of the inaccuracies of the two clock generators, for example (2*0.8 .mu.s)=1.6 .mu.s.
The wandering off (drifting) of the local clock generators of the individual drives between 2 successive synchronization clocks of the ring manifests itself as an additional synchronization error, since the individual drive controllers execute several control cycles during one data transmission cycle, of for example 8 ms.
In the case of a control cycle in the drive of, for example, 250 .mu.s and a data transmission cycle of, for example, 8 ms on the ring, the drive executes 32 control processes during one data transmission cycle. Only the first control process is in this case strictly synchronized with the synchronization clock of the ring. In the case of the following 31 interpolating control processes, the time control is carried out via the local clock generator of the drive. The inaccuracy of the local clock generators of the individual drives manifests itself as additional synchronization errors in the interpolating control processes.
With an increasing number of drives in the ring, the cycle time of the data transmission therefore increases, and hence the time interval of successive synchronization clocks, and with an increasing time interval between successive synchronization clocks, the wandering apart (drifting) of the local clock generators (crystals) of the individual--drives increases. Hence the synchronization error increases and the precision of the position control no longer achieves the required values.
The drive controllers connected to a fast drive bus obtain not only the common synchronization clock, but also the desired value data from the central drive control means, which is also the master during the data transmission. With an increasing number of drives, the time needed for the desired value calculations and desired value transmissions increases. With an increasing number of drives, the loading of the central drive control means increases as a result of the cyclic desired value calculations. The central drive means supplies the connected drives cyclically with new individual desired values and with a common synchronization clock. The cycle times for the desired value calculations and the common synchronization clock are preferably of the order of magnitude of 1 ms.
With an increasing number of connected drives, the time expenditure for the desired value calculations in the central drive control means increases. For example, given a computing time of 250 .mu.s for the desired values of one drive, and given 32 connected drives, the cycle time of the desired value calculations in the drive control means must be at least 8 ms. This constitutes an enormous computing load on the central drive control means which, for its part, once more limits the number of connected drives.
With an increasing number of drives, the effects of an individual error in the central drive control means or in the drive bus increase.
The ring-like glass fiber connection according to the SERCOS standard is not redundantly designed, and the drive control means, which is simultaneously the master in the data transmission, is also not redundantly designed. In the case of an error in the central drive control means, or in the case of an error in the drive bus, all the connected drives thus fail.
In industrial production plants it is often required to limit the effects of individual errors to a closely limited environment. An individual error in the electronics may lead to the failure of a specific functional unit but in no way to the failure of an entire production plant.
In printing machine construction, it is for example largely tolerated that an individual error in the drive electronics leads to the failure of one functional unit, for example of one printing unit having 8 printing cylinders. It is not tolerable for an error in the drive control means or in the drive bus to lead to the failure of an entire production plant, for example of an entire newspaper printing plant.
The number of drives to be connected to a drive control means and a drive bus should therefore preferably be limited to a specific number on availability grounds, so that failure of the drive bus or of the drive control means has effects only on one individual functional unit of an industrial plant, for example on one printing unit of a newspaper printing plant.
A centralized drive system having a central drive control means and a drive bus, to which all the drives to be operated in precise synchronism are connected, often does not correspond to the natural structure, functional distribution and module formation in large technical plants.
Industrial plants often comprise several self-contained functional units, which in each case contain all the associated mechanical and electrical functions.
Control systems and drive systems are therefore preferably structured, associated and distributed in accordance with the functional units of the industrial plant. This results in self-contained functional units which can be tested and commissioned simply and independently of one another. The interfaces between functional units which are delimited in this way are simple and comprehensible.
The advantages of a decentralized, distributed structure--capable of being adapted to the technical plant--of the control and of the drive system result in particular from the clearer system structure, simpler comprehensibility, better testability, closely delimited error effects. These advantages often lead to lower production costs, operating costs and maintenance costs.
In the case of newspaper printing presses, for example, the printing units, folding apparatus and reel carriers are preferably designed as self-contained functional units and are in each case equipped with dedicated, local control means and dedicated, local drive systems.
A centralized drive system having a central drive control means is a great obstacle to the implementation of technical plants having self-contained functional units and simple, clear interfaces.
The significant disadvantage of a central drive control means is that all the desired value data has to be led to the individual drives via the central drive control means. It is not possible for the local control means of a functional unit to communicate directly with the local drive control means of the functional unit, since there are no decentralized, local drive control means of functional units.